In 3GPP there is an initiative ongoing to define a GPRS enhancement known as the One Tunnel Solution (OTS). The method was proposed already in 3GPP Release 4 and elaborated in the TR 23.873 (see FIG. 1a). The TR proposes to remove the user plane from an SGSN (Serving GPRS Support Node) and send pay load directly between a GGSN (Gateway GPRS Support Node) and an RNC (Radio Network Controller). The control plane is still implemented in the SGSN node, which becomes a pure control signalling node without user plane. However, the proposal had certain shortcomings, e.g. the solution was not designed for roaming mobiles. For roaming, the user plane will be connected through the SGSN in the visited network (NW) towards the GGSN in the Home PLMN (Public Land Mobile Network), which means that the user plane can not be removed entirely from the SGSN.
To overcome the roaming problem an alternative architecture was proposed by Nortel, involving the so called GGSN Bearer Relay in the V-PLMN (see FIG. 1b). With this solution the user plane will in the roaming case pass through the visited GGSN and the RNC, without passing through the SGSN. The SGSN will be a pure control node. In the GGSN Bearer Relay solution, the control signaling for roaming users will be sent on an ordinary Gp interface between the V-SGSN and the Home-GGSN. Also there will be a Gn interface between the SGSN and the GGSN in the visited network. For a roaming user these two control interfaces will be used simultaneously for signaling to the H-GGSN and to the V-GGSN.
The GGSN Bearer Relay for the One Tunnel Solution solves some but not all problems of the original one tunnel Solution in Release 4.    1. There will be a security problem in the GGSN Bearer Relay. When the Gp interface is used for roaming users, the topology of the Visited PLMN will be openly visible outside the V-NW. The SGSN nodes and their IP addresses are exposed to the GRX and external networks in addition to the GGSN Proxy's. External control signaling is terminated in several less protected points deep inside the network, instead of in a few well protected points at the edge of the NW.    2. Firewall configurations are complex when CP (control plane) and UP (user plane) are split. Stateful firewalls are not possible since ISRAU (Inter SGSN Routing Area Update) will be reported outside of the V-PLMN.    3. PLMN Roaming problem. There is an issue in the GGSN Bearer Relay with MM (Mobility Management) procedures when roaming between different PLMN's. This is due to that the GGSN in the old VPLMN shall not be used anymore, and needs to be replaced with a GGSN in the new VPLMN.    4. Another problem with the current release-6 3GPP architecture is that PS (Packet Switched) based traffic, e.g. IP Telephony, is always routed to the Home-PLMN. There is no way to break out traffic locally in the Visited-PLMN. The GGSN Proxy provides an alternative for terminating PS traffic in the VPLMN. With a specific implementation in GGSN Proxy, a PDP Context may be forced to terminate in VPLMN. This may for example be very important when the PS domain shall support Emergency IP Telephony, where the caller needs to be connected to a local emergency call center in the VPLMN.